ZOO501 Final Term Important Questions Spring 2025 by Muhammad Zaman

Protostomes

1. The Ecdysozoa includes those animals that molt. Its major constituent is
2. The second major group of protostomes are the Lophotrochozoa. They are characterized by a common type of cleavage (spiral), a common larval form, and a distinctive feeding apparatus.

These phyla include annelids, molluscs, and flatworms.

Deuterostomes

•  From the Greek term meaning "mouth second."
•  In this group, the anus forms first from the blastopore, followed by the mouth.
•  Includes phyla like chordates (e.g., vertebrates) and echinoderms (e.g., starfish).

Three basic modes of commitment

The three basic modes of commitment in embryonic development are:

1. Autonomous Specification:

Cells acquire specific fates based on the differential distribution of cytoplasmic molecules in the egg.

•  Characteristic of most invertebrates, leading to "mosaic" development where cells cannot change their fate if removed.
•  In autonomous specification, certain cytoplasmic molecules present in the egg determine the fate of cells.
•  This type of specification is common in most invertebrates.
•  Cell type specification happens before any large-scale embryonic cell migration.
•  This autonomous specification leads to a pattern of development called mosaic development. It’s like the embryo is made up of independent self-differentiating parts, just like a mosaic.
•  Invariant cleavages occur, producing the same lineages in each embryo of the species.
•  The fate of blastomeres are invariant.
•  Produces “mosaic” (“determinative”) development: cells cannot change fate if a blastomere is lost.

2. Conditional Specification:

•  Cells' fates depend on interactions with neighboring cells, making this mode regulative.
•  Typical of vertebrates; cells can alter their fate if others are removed, allowing for compensation (e.g., identical twins).
•  Conditional specification is a characteristic found in all vertebrates and a few invertebrates.
•  o In this type of specification, cells interact with each other to determine their fate.
•  o The relative positions of cells become important in this process.
•  o Unlike in autonomous specification, the fate of cells is not predetermined and can vary.
•  o Variable cleavages occur, meaning that cells do not have fixed assignments.
•  Additionally, there are significant cell rearrangements and migrations that happen before or during specification.
•  o Capacity for "regulative" development: allows cells to acquire different functions.

3. Syncytial Specification:

•  Found in many insects, where interactions occur between different parts of a single multinucleate cell (syncytium).
•  Insects like Drosophila use mRNA gradients (e.g., Bicoid and Nanos proteins) to specify body regions.